Prevent showing divider if launcher was never shown

* With stashing recents animation gets cancelled
almost instantly, so there's no point trying to
"show" the divider bar if it was never hidden
* We can use launcher showing as a good enough
proxy for whether the divider bar was ever hidden
to begin with

Fixes: 230825541
Test: Stashing/unstashing didn't cause crash
Change-Id: I3abc4ecf3605a66e67db971ae14aa6ec493d8f32